You should be able to do such a check with a couple vector dot products. Take the view vector of each and dot them. If they are both positive the guys are facing a similar direction. Compare the value to the angle tolerance you wish to enforce. Also if you dot the victims forward vector with the vector to the attacker you can discover if the attacker is behind, as it will be negative, and again you can tweak the angle tolerance by comparing it to the angle tolerance you want to allow.